Support new dataset API

Arkindex 1.5.4 introduces changes to the ListProcessDatasets endpoint. We need to support these changes in the next Base-Worker release.

We will change how dataset are loaded and stored.

The new payload is

{
    "dataset": {...}, # This should be used to create a arkindex_worker.models.Dataset instance
    "id": "...",
    "sets": [] # This should be stored on the dataset instance, as `selected_sets` attribute
}

The change should be done in arkindex_worker.worker.dataset.DatasetMixin.list_process_datasets.

The new selected_sets attribute should be used when listing the elements of a dataset, filtering on the set.